1. Field of the Invention
The present invention relates to a display device including a thin display panel. More particularly, the present invention relates to a reinforced structure of a display device in which a display panel is sandwiched between a plate member and a protective plate or a touch panel disposed on a display surface side. For example, the present invention is applicable to portable electronic devices such as a cellular phone, a PDA, or an electronic dictionary, and a display device thereof. Examples of the thin display panel include a liquid crystal display device of a passive matrix system, which uses an STN or TN liquid crystal, a liquid crystal display device of an active matrix system that drives the TN liquid crystal by a TFT element in a VA mode or an IPS mode, an organic EL display, and a plasma display.
2. Description of the Related Art
A liquid crystal panel has been known as a typical thin display panel. The liquid crystal panel has a structure in which two glass substrates each having a color filter or a TFT formed thereon are bonded to each other with a sealing agent provided on an outer periphery of the glass substrates, and liquid crystal is sealed in a gap between those glass substrates. Drive signals are applied to electrodes disposed on the surfaces of the glass substrates to drive the liquid crystal. The drive signals are supplied by a driver IC mounted face-down on the glass substrates by an anisotropic conductive film. The liquid crystal panel controls liquid crystal molecules according to an ON signal or an OFF signal contained in the drive signal, and controls the absorption and transmission of light by means of absorptive polarizing plates disposed on the front surface and the rear surface of the liquid crystal panel for display.
The display systems are classified into a reflection type and a transmission type. In the transmission type, a backlight is disposed behind the liquid crystal panel. The backlight is classified into a side light system in which a light source such as an LED or a cold cathode fluorescent tube is disposed at a side surface of a light guide plate, which surface-emits a light therefrom, and a direct type system in which the light source is disposed directly below the display surface, and the light is diffused with a diffuser into a uniform light to conduct surface emission. Further, in order to improve the luminance, a diffusion sheet or a lens sheet may be disposed. The lens sheet has a structure in which a plurality of fine lenses are arranged linearly in parallel to each other, and has a function of condensing the light in one axial direction. In order to condense the lights in an x-axis direction and a y-axis direction, two lens sheets may be disposed in lamination. When the pitches of prisms of the lens sheets interfere with the pixels of the liquid crystal panel to generate a moiré pattern, there is employed a method involving changing the pitches of the lenses. The diffusion sheet is configured such that ink containing fine particles is formed on the surface of a transparent film.
In order to efficiently emit the light, a sheet having a high reflectivity of light, such as a sheet on which silver is deposited is disposed under the light guide plate of the side light system or under a fluorescent tube of the direct type system. As a method of fixing the backlight and the liquid crystal panel to each other, there has been generally used a light shielding double-faced tape having a thickness of about 0.02 to 0.10 mm. The light shielding double-faced tape is disposed in the outer periphery of the display portion of the liquid crystal panel, to thereby fix the liquid crystal panel onto the backlight. The light shielding double-faced tape used in this example is a double-faced tape which is white on the backlight side and is black on the liquid crystal panel side. As a method of improving the impact resistance, there is given a method in which an elastic member having light scattering particles is used as a shock absorbing member, and the rear surface of the liquid crystal panel and the light emission surface of the backlight are attached firmly to each other (for example, refer to JP 2999369 B).
Further, there has been known a structure in which a concave portion into which the diffuser or the lens sheet are incorporated is defined in the light guide plate of the backlight, the light shielding double-faced tape is disposed on the outer periphery of the light guide plate, by which a film is bonded to configure the backlight, and the film and the liquid crystal panel are bonded firmly to each other, to thereby improve the thrust strength (for example, refer to JP 2004-347957 A).
In the actual product, a protective plate, a transparent touch panel, or the like is disposed as a transparent member on the liquid crystal display surface. The protective plate is made of acrylic such as PMMA or hardened glass. A metal thin film made of chromium or nickel and a color print such as black may be laminated on the outer periphery of the protective plate to form a window frame. Further, the window frame may be formed by means of only color printing. In general, a hard coating for damage prevention is formed on the surface of the PMMA. In case that the glass thickness is 10 mm to 0.5 mm, chemically hardened glass is frequently used as the hardened glass. In particular, when the glass thickness is thicker, an air-quench hardened glass is adopted. The touch panel is classified into an analog resistive film system, a digital resistive film system, a capacitance system, an ultrasonic system, or an electromagnetic induction system. In the case of the electromagnetic induction system, a glass substrate for damage prevention may be bonded onto the display surface of the display panel. Similarly, in the capacitance system, the hardened glass may be bonded onto the display surface.
The transparent member and the display panel are bonded and fixed to each other by the frame-like double-faced adhesion tape disposed on the outer periphery of a display area. Alternatively, a transparent adhesive or a transparent adhesion sheet is disposed on the entire area including the display area so that the transparent member and the display panel are bonded onto each other over the entire surface.
Further, as structures where the display device including the transparent member on the display surface side is housed in an electronic device, there are given a structure in which the display device is held from the upper portion of the outer periphery of the transparent member with an aid of an exterior case including an opening at a display portion, a structure in which the transparent member is provided with a step, and a lower level of the step is held by the exterior case, and a structure in which the transparent member includes an outer peripheral portion for fixing, which is larger than the display panel, an adhesive sheet is bonded onto the rear surface of the outer peripheral portion, and the outer peripheral portion is bonded and fixed to the electronic device.
As the thinned portable device such as a cellular phone is increasingly desired, it is necessary to made thin the glass substrate that constitutes the display panel, and to thin the light guide plate of the backlight of the side light type. For example, in the liquid crystal panel, the thinning is advanced to such an extent that the thickness of the glass substrate is 0.10 to 0.15 mm, and the thickness of the light guide plate is 0.25 mm. However, because those members are thinned, the glass substrate is frequently broken due to an impact caused by dropping or the like. Under the above-mentioned circumstances, in order to prevent the glass substrate from being broken, there is adopted a system in which a hardened glass is used for the protective plate on the display surface side, or the protective plate is bonded to the entire display surface of the liquid crystal panel with an aid of an optical adhesive or an optical sheet.
However, there are many cases in which a case of the main body of a cellular phone and the like is largely deformed due to the impact caused by dropping or the like, and the liquid crystal panel is damaged due to the deformation. Under the above-mentioned circumstances, the material of a frame of the backlight is changed from plastic to a material that is high in Young's modulus, such as magnesium. However, the breakage of the liquid crystal panel due to the impact, in particular, the breakage that occurs in the glass substrate on the rear surface side upon receiving damage from a part which is disposed on the back side can be hardly reduced.
Further, in the structure where the liquid crystal panel and the light guide plate of the backlight are bonded tightly to each other with an aid of the elastic member, there arises such problems that peeling is liable to occur due to thermal expansion at high temperature or at low temperature, and the luminance is deteriorated because the optical sheet such as the lens sheet cannot be provided.
In order to prevent the peeling, there is proposed a structure in which the rear surface of the liquid crystal panel is bonded directly to the entire light emission surface of the light guide plate by the optical adhesive. In this structure, the optical adhesive exists between the optical sheet provided on the front surface of liquid crystal panel and the light guide plate. Because the refractive index of the optical sheet is close to the refractive index of the optical adhesive, the optical function of the optical sheet is deteriorated, and the luminance of the display surface is deteriorated. Even in a structure where the rear surface of the liquid crystal panel is bonded to the entire surface of the light guide plate without the optical sheet, the luminance is deteriorated as compared with the backlight using the optical sheet. In any cases described above, the optical disadvantage occurs.
Further, when the rear surface of the liquid crystal panel is bonded directly to the light guide plate by the optical adhesive, the strength is improved with respect to an impact in the pushed direction, but the seal of the liquid crystal panel is peeled off when an impact is applied in the peeling direction.
In the electronic device using the display device, when the display device with a structure in which the transparent member is bonded onto the display panel is held by the exterior case from the outer peripheral upper surface, the electronic device is thickened by the thickness of the exterior case. In this case, the electronic device is limited in terms of design. Further, even in the structure where the transparent member is provided with a step, it is necessary to thicken the transparent member in order to form the step. In the structure where the transparent member is enlarged, and the adhesive sheet is disposed on the rear surface, an “adhesive fixation overlap width” that is equal to or higher than a given width is required. This leads to such a problem that the frame becomes larger.